The invention relates to a system for making dendritic silicon webs and more particularly to a system for maintaining the molten pool from which the dendritic web is pulled at a constant level by adding material at the desired controlled rate.
Commercialization of the dendritic web growth process requires replenishment of the melt pool to produce long ribbons of high quality. Although crystals as long as 7 and 8 meters can be grown without replenishment during growth, the time required for replenishment between crystals adversely affects the output and efficiency of the process. Commercial viability of a process for dendritic web silicon for photovoltaic applications requires essentially continuous growth of the ribbons. Thus, the silicon melts from which the ribbons crystals are grown must be continuously replenished not only to replace the material removed from the melt, but also to maintain the thermal conditions in the growth system. The position of the melt surface with respect to the lid that covers the crucible especially affects the thermal conditions in the vicinity of the juncture of the dendritic web and the molten pool.
U.S. Pat. No. 4,389,377 describes a melt system including a susceptor-crucible assembly having improved gradient control when melt replenishment is used during dendritic web growth. The improvement lies in the formation of a thermal barrier in the base of the receptor which is in the form of a vertical slot in the region of the susceptor underlying the crucible at the location of a growth compartment and a melt replenishment compartment. The result achieved is a step change in temperature gradient in the melt thereby providing a more uniform temperature in the growth compartment from which the dendritic web is drawn.